A 32-year-old man recently noticed a dark lesion behind his right knee. The patientdenies trauma and fever and has no other medical problems. Since he started workingfor the parks and recreation department of the state of Florida 18 months ago, hehas spent an increased amount of time outdoors.
This well-nourished man is in no acute distress. Temperature is 37.2C(99F); heart rate, 88 beats per minute; respiration rate, 17 breaths per minute,and blood pressure, 132/83 mm Hg. Heart rhythm is normal and lungs are clear.Examination of the lower extremities reveals a 2.5 cm focal area of hyperpigmentationin the right popliteal fossa that is heterogeneous in appearance. Theedges of the lesion are slightly raised. There is no evidence of surrounding erythema,and palpation of the right popliteal fossa and the right inguinal region revealsno adenopathy. Pulses in the lower extremities are strong from the inguinal regionto the foot. Strength and sensation in both lower extremities are also normal.An excisional biopsy of the lesion reveals melanoma. The lesion is classifiedas Clark level IV, and tumor thickness is 1.8 cm.To determine whether regional adenopathy is present, which diagnostic testwould you order and why?WHICH TEST AND WHY: A nuclear medicine lymphoscintigram is extremely accurateand identifies the correct nodal basin in more than 95% of patients withmelanoma. It is extremely difficult to predict nodal drainage patterns solely onthe basis of anatomic charts. In truncal melanomas, there is a discordance of32% to 62% between the site of the nodal basin predicted by anatomic chartsand the actual site, as confirmed bylymphoscintigraphy. With head andneck lesions, the discordance is 63%to 84%.Lymphoscintigraphy is performedby hand-injecting sulfur colloidaround the lesion intradermally inorder to introduce tracer into the lymphaticsystem, where it typically followsnormal drainage pathways. Thefirst lymph node in which the traceraccumulates is the sentinel lymphnode. A surgeon can then remove thisnode for pathologic examination. Ifthe examination results are normal,no further dissection is needed. Ifthey are abnormal, a formal radicaldissection of the lymph node basin isusually performed.Results of the study. In animage from this mans lymphoscintigram,tracer activity is localized inthe popliteal fossa at the site of injection (Figure 1). From there, a "line" projects toward theinguinal region. A subsequent transmission image(which is created by superimposing on the nuclear imagea crude radiograph that is obtained by placing an x-raysource behind the patient during the nuclear medicineexamination) shows localization of the tracer in the inguinalregion (Figure 2). Another, slightly delayed transmissionimage confirms that the sentinel node is locatedin the right inguinal region (Figure 3).Outcome of this case. The sentinel node was removedand was found to be normal. As a result, the melanomawas classified as stage IIA. Lymphoscintigraphyspared this patient a radical dissection and such associatedpotential complications as lymphedema, pain, seromaformation, infection, and paresthesias. At 1-year follow-up,he was disease-free.Lymphoscintigraphy in other settings. Here lymphoscintigraphyhelped identify the patients sentinel nodein the correct basin. However, the correct basin maynot always be obvious. In addition, there is often morethan one nodal basin that drains a region. This is particularlytrue in the head and neck region, where 2 sentinelnodes and sometimes 3 are frequently identified. Forexample, a transmission image from a lymphoscintigramin a patient with a melanoma on his back directly overthe midline in the thoracic region shows 2 nodal basinsdraining the region (Figure 4). Consequently, 2 sentinelnodes are identified.